Laser machining of electroactive ceramics

ABSTRACT

Laser beam machining is applied to form arbitrarily shaped electroactive ceramics for transducers (e.g. electromechanical sensors and actuators). One particularly preferred embodiment of the invention comprises machining parallel grooves in a ceramic plate to improve flexibility. The grooves provide strain relief in bending by relieving Poisson strains transverse to the direction of bending. This embodiment offers the further benefit that planar anisotropy or directionality is introduced in the transducer. The machining process of the invention further enables the production of more complex geometries than those currently known in the art. Because of the flexibility of the machining process, virtually any desired transducer shape may be produced.

FIELD OF THE INVENTION

[0001] This application relates generally to methods of formingelectroactive ceramics by laser machining, and more particularly tomethods of producing sensors and actuators having superior mechanicaland electroactive properties.

BACKGROUND OF THE INVENTION

[0002] Ceramic materials are brittle and are difficult and costly toform in arbitrary shapes. In particular, it would be desirable to formelectroactive ceramics in more complicated shapes than those currentlyavailable.

[0003] Some methods of machining ceramics for a variety of reasons havebeen disclosed. For example, U.S. Pat. Nos. 4,131,484 to Caruso et al.,U.S. Pat. No. 4,642,505 to Arvanitis, and U.S. Pat. No. 5,369,862 toKotani et al. disclose using a laser to adjust the resonant frequency ofa piezoelectric ceramic. U.S. Pat. Nos. 4,422,003, 5,615,466, 5,796,207,and 5,818,149, all to Safari et al., describe methods of producingpolymer-ceramic piezoelectric composites. U.S. Pat. No. 4,650,619 toWatanabe describes a method of laser machining apertures in a ceramicmember to create gas supply passages.

[0004] It is an object of the present invention to provide a method ofmachining electroactive ceramics which is relatively inexpensive,capable of producing complex shapes, and does not unduly compromise theelectroactive properties of the electroactive ceramic. It is a furtherobject of the present invention to provide sensors and actuators havingflexibilities and anisotropic behaviors superior to those known in theart, as well as improved mechanical robustness and handling properties.It is still a further object of the present invention to provide sensorsand actuators having shapes which allow superior electromechanicalperformance compared to those known in the art. It is yet a furtherobject of the present invention to provide sensors and actuators whichcan be attached to electrodes in improved configurations.

SUMMARY OF THE INVENTION

[0005] The present invention achieves these and other objects byproviding a method of machining electroactive ceramics for transducerapplications. A Laser-Beam Machining (LBM) process is used to removematerial selectively enable cuts, grooves and general forms fortransduction. For example, a particularly useful form will be forsurface strain relief of planar electroceramics. This introduces greaterbending flexibility as well as directional or anisotropic couplingbehavior in the material. Other example usages include relief patternssimilar to those developed in silicon wafers for Micro ElectroMechanicalStructures (MEMS). The LBM process includes through-cuts, grooves andother material removal from a ceramic substrate, and is not necessarilylimited to planar structures. This process offers a cost-effectivealternative for moderate to large-scale production of flexible,anisotropic sensors and actuators.

[0006] In one aspect, the invention includes a method of producing anelectromechanical device, by poling an electroactive ceramic, lasermachining the ceramic into a desired shape, and incorporating theceramic into an electromechanical sensor or actuator. The lasermachining may include, for example, machining grooves or slots in theceramic, which may serve to render the properties of the ceramicanisotropic. The sensor or actuator may be a substantially planar,stress-relieved transducer. The electroactive ceramic may be, forexample, a piezoelectric or electrostrictive ceramic. Poling may beachieved either before or after laser machining. Small or large amountsof material may be removed by machining, for example 1%, 5%, 20%, 50%,75%, or 90% of the electroactive ceramic. The surface area of theceramic may be increased by 10% or more by the machining process.

[0007] In another aspect, the invention comprises an electromechanicaldevice comprising a substantially planar electroactive ceramic havinggrooves defined on a surface thereon, the grooves allowing the ceramicto conform to a curved surface, for example a surface having a radius ofcurvature of 0.25″. The device may be, for example, an electromechanicalsensor or actuator. Parallel grooves may allow the device to conform toa cylindrical surface, or concentric grooves may allow the device toconform to a spherical surface.

[0008] In yet another aspect, the invention comprises anelectromechanical device comprising a substantially planar bimorphelectroactive ceramic member. The member may have slots defined thereinthat allow multiplication of an electromechanical bending response ofthe bimorph member. The device may be, for example, an electromechanicalsensor or actuator. The slots may be substantially concentric,substantially parallel, or in any other suitable geometry.

BRIEF DESCRIPTION OF THE DRAWING

[0009] The invention is described with reference to the several figuresof the drawing, in which,

[0010]FIGS. 1a-1 d illustrate some of the many transducer shapes whichmay be achieved by the methods of the invention;

[0011]FIGS. 2a-2 d illustrate the process of machining a wafer andincorporating it into a composite transducer;

[0012]FIGS. 3a-3 c illustrate how a transducer according to theinvention may be used on a cylindrical surface;

[0013]FIG. 4 illustrates a displacement amplifying device according tothe invention;

[0014]FIG. 5 shows a cross-section of two displacement amplifyingdevices as shown in FIG. 4;

[0015]FIGS. 6a-6 b illustrate other displacement amplifying devicesaccording to the invention; and

[0016]FIG. 7 illustrates a radially patterned actuator according to theinvention.

DETAILED DESCRIPTION

[0017] It is well known that ceramic materials are brittle and aredifficult and costly to form in arbitrary shapes. Laser-beam machining(LBM) has been used for removing material from ceramics. In the presentinvention, this technique is applied to form arbitrarily shapedelectroactive ceramics for transducers (e.g., electromechanical sensorsand actuators). One particularly preferred embodiment of the inventioncomprises machining parallel grooves in a ceramic plate to improveflexibility. The grooves provide strain relief in bending by relievingPoisson strains transverse to the direction of bending. This embodimentoffers the further benefit that planar anisotropy or directionality isintroduced in the transducer. This embodiment may thus serve as a lowercost replacement for the polymer/ceramic composites disclosed, forexample, in U.S. Pat. Nos. 4,422,003, 5,615,466, 5,796,207, and5,818,149.

[0018] The LBM process further enables the production of more complexgeometries than those currently known in the art. Because of theflexibility of the machining process, virtually any desired transducershape may be produced. Examples of shapes that may be produced by theprocess are shown in FIGS. 1a-1 d.

[0019] This process may be applied to any type of electroactive ceramicmaterial in any base form. The material can be processed with or withoutelectrodes, and before or after any polarization process. The LBMprocess removes ceramic material in any arbitrary topography, includingthrough-cuts, and three-dimensional relief patterns.

[0020] The process may be used in combination with other existingcompositing techniques and packaging technology. Such techniques includethe incorporation of polymer or other matrix materials, other passive oractive phases, and/or flexible circuit layers. The flexible circuitlayers may be used for electrical insulation, electrical connectionpoints, and mechanical protection, as has been demonstrated withelectroceramic wafers and composites containing fibrous or rod forms.Structural reinforcing materials may also be incorporated in thecomposites. Existing concepts of shaping for spatial filtering may alsobe applied, with the added benefit of thickness tailoring. Thistechnology will also enable more complex geometries for electroceramicbimorph applications. As with MEMS technologies for etching silicon, theLBM process is used to design relief patterns in electroceramics.

[0021] The laser-machined electroceramics of the invention are expectedto be particularly useful for electromechanical sensors and actuators.Actuators, for example, may be devices incorporating piezoelectric,electrostrictive, or magnetostrictive materials, which convertelectrical or magnetic energy into mechanical energy. Electromechanicalsensors generally use the same principles to convert mechanical energyinto electrical or magnetic energy, thereby providing feedback aboutstrains within a material.

[0022] The LBM Method: Laser-beam machining is a process in which highlyfocused, high-density energy melts and/or evaporates material from asubstrate. The laser can be controlled to remove material in a desiredlocation. Similar to a more traditional machining process or a carvingprocess, the substrate or laser can be displaced in order to rapidlyremove material in a prescribed manner. The depth of the materialremoved can also be controlled. Localized heating of the material mustbe considered while processing to limit the size of the heat-affectedzone. While LBM has been used on a small scale for trimming resonatorsto adjust their frequency, it has generally been believed that thismethod would destroy the electroactive properties of the ceramic in theheat-affected zone. The present inventors have surprisingly discoveredthat this effect can be minimized or eliminated, and have demonstratedthat an electroactive ceramic may not even require repoling aftermachining.

[0023] The LBM process has been implemented with a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser. The laser has a variableaperture, variable power, and variable pulsing which can be used tocontrol the removal of material. In addition, scanning mirrors are usedto precisely deflect the laser beam and control its position on theceramic surface within a fraction of a mil. This laser has a standardbeam width of 0.004″, which can be adjusted to as low as 0.001″ withsome reduction in power. Forms with features on the order of 0.002″ insize, and curves with a radius of approximately 0.002″ should beachievable with this laser. By varying the rate of translation and thelaser power and pulsing variables, the effective cut width and depth canbe controlled. Cut depths ranging from 1% to 100% of the wafer thicknesshave been achieved, for wafers having thicknesses in the range of 0.005″to 0.020″. Using a different laser system should enable furtherexpansion of the possible designs which can be produced. A system with asmaller beam width and precise scanning controls can enable themachining of smaller features in the ceramic.

[0024] Example Analysis for Planar Forms: For a planar electroceramicform, typically referred to as a wafer, the benefit of the LBM processcan be clearly illustrated. It should be noted that this is only onespecific example of the potential applications of the method describedherein which is applicable to more general forms. The process isillustrated mechanically in FIGS. 2a-2 d. The wafer 10 has a thin plategeometry, which is essentially isotropic. Given a rectangular plate, theprocess may be applied to remove material along multiple parallel lines,oriented along the longitudinal axis. This will form multiple grooves 12in the ceramic material 10, which are not complete through-cuts. Thusthe ceramic remains in one continuous piece. FIG. 2a depicts across-section prior to machining, and FIG. 2b depicts the cross-sectionafter the grooves have been cut.

[0025] Next consider the effective stiffness of the resulting ceramicform or electroceramic device 10. The ceramic material is essentiallydivided into parallel strips 14 having the original thickness of theplate and joined by strips of reduced thickness 16. Now consider thelimiting case in which the thickness of the thinner strips 16 is setarbitrarily close to zero. Macroscopically, the ceramic material now canbe considered as multiple parallel strips of ceramic spaced apart withair gaps in between. Clearly, the longitudinal stiffness of the ceramicdevice, measured along the length of the strips, and the transversestiffness, measured across the strips and air gaps, will not be equal aswas initially the case with the wafer. The device may be considered as acomposite consisting of ceramic strips and strips of air in between.Using a Mechanics of Materials approach, the effective stiffness of thedevice along the longitudinal axis will be reduced by the ratio of thecross-sectional area of ceramic to the total initial area since thestiffness of the air gap is negligible. Along the transverse axis, theeffective stiffness will be zero since any applied strain on thematerial will result in zero force. The dimensions and spacing of thegrooves cut into the ceramic can be set to achieve a desired reductionin the transverse stiffness. The longitudinal stiffness will be muchless sensitive to this selection.

[0026] While the microscopic electromechanical coupling behavior willremain substantially unchanged in the machined ceramic form, themacroscopic behavior will differ significantly as a result of thestiffness anisotropy. In a typical application for actuation or sensing,a planar electroceramic may be bonded to or embedded within a planarhost structure. Mechanical deformation and stresses in the structure arecoupled to those in the electroceramic through the bond layer, similarto a multi-layer composite laminate. As a result of the anisotropicstiffness described in the limit case above, a structural deformation inthe host structure along the transverse axis of the device will resultin no electromechanical coupling, as a result of the negligibletransverse stiff niess. A macroscopic deformation of the device formwill result in no actual strain in the electroceramic material, and thusno coupling from mechanical energy to electrical energy. However,coupling will remain along the longitudinal axis of the device. Thisform of anisotropy in an transducer can be used advantageously in thedesign of active structures.

[0027] This example can also be used to illustrate additional benefitsof such a geometry. The same device can be combined with a second phase18, such as a compliant matrix material, to form a composite, shown inFIG. 2c. Alternatively, the grooves may be filled with an electrodematerial to provide a strong mechanical and electrical connectionbetween the electroactive ceramic and the electrode. Optionally, thecomposite may be further packaged, as shown in FIG. 2d. As has beendemonstrated in prior art, a composite form consisting of relativelystiff fibers in a compliant matrix offers the advantage of increasedspecific strength and toughness as compared with a monolithic form ofthe fiber material. In this example, the machining parameters may be setto achieve a geometry similar to that of a fibrous composite. Theprocess can also be considered as a strain- or stress-relief process forthe ceramic material where transverse stresses are relieved duringlongitudinal bending. Thus the LBM process can be used to effectivelyproduce a composite transducer device.

[0028] A further benefit of the device shown in FIG. 2b as compared tofibrous composites is that its handling characteristics aresignificantly improved. Fibers of electroactive ceramics are brittle anddifficult to align and form into a composite. Crossed fibers breakeasily when a fiber preform is compressed or otherwise stressed. Inaddition, currently available fibers are more porous than bulk material,and are difficult to make in a uniform size, leading to significantproblems when attaching electrodes. In addition, many fibers exhibitsignificant curvature. These characteristics make it difficult toachieve a quality fibrous composite actuator or sensor. In contrast, thesensors and actuators of the invention are mechanically robust, and itis relatively easy to predict and control their electromechanicalproperties.

EXAMPLES

[0029] The LBM technique has been successfully applied to a planarelectroceramic wafer. The material was a Lead Zirconate Titanate(PZT-5A) which was initially electroded and poled through the thickness,as received from the manufacturer. A number of parallel grooves weremachined into one surface of a 3 inch square wafer which was 0.005inches thick. The grooves were approximately 0.005 inches wide and werespaced evenly approximately 0.020 inches apart. Beam width was 0.004inches; slightly more material was removed because of heat transfer inthe ceramic. Actuation testing of the test article after machiningshowed no degradation in induced free strain capability or polarization.In addition, an in-plane anisotropy was measured such that lower strainlevels were recorded transverse to the axis of the grooves and greaterflexibility was also observed, allowing the wafer to be used, forexample, on the surface of a cylinder, as shown in FIGS. 3a-3 c. Theresults indicate that the ceramic material surrounding the machinedareas (the heat-affected zone) was not significantly degraded and thatthe strain relief concept was viable. It will be apparent to thoseskilled in the art that grooves may be positioned to allow the wafer toconform to other shapes. For example, concentric circular grooves may beused to allow the wafer to conform to a spherical surface.

[0030] A second unique design which can be achieved by the LBM processis illustrated in FIG. 4. That figure shows a round bimorph displacementamplifying device. In a bimorph device, electroceramic material is usedin a two-layer configuration. The upper and lower layers are actuatedopposingly so that an extension in one layer is commanded simultaneouslywith a contraction in the other. The net effect is a bending actuation.The LBM process can be used to cut circular ring shaped holes in aceramic bimorph disk in the pattern shown in FIG. 4, enabling largeout-of-plane deflections. FIG. 5 shows a cross-sectional view alongsection A-A of a mirrored pair of actuated disks, which act together tofurther increase the displacement capability. Those skilled in the artwill perceive that the pattern of shaped holes may be changed to alterthe response of the actuator. For example, a pattern of parallel, ratherthan concentric, slots may be preferred in certain applications, inparticular for producing a “bending” actuator, as illustrated in FIGS.6a. A relating “twisting”configuration which is expected to achieveimproved flexibility is shown in FIG. 6b. Still another actuator isshown in FIG. 7; this radially patterned transducer may be used foracoustic actuation, for example.

[0031] Other embodiments of the invention will be apparent to thoseskilled in the art from a consideration of the specification or practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with the true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method of producing an electromechanical device, comprising poling an electroactive ceramic; laser machining the electroactive ceramic to form a desired shape; and incorporating the electroactive ceramic into an electromechanical sensor or actuator.
 2. The method of claim 1, wherein laser machining includes machining grooves into a surface of the electroactive ceramic.
 3. The method of claim 2, further comprising depositing an electrode material into the grooves in the surface of the electroactive ceramic.
 4. The method of claim 1, further comprising depositing an electrode material onto a surface of the electroactive ceramic produced by laser machining.
 5. The method of claim 1, wherein the produced sensor or actuator is a strain-relieved, planar transducer.
 6. The method of claim 1, wherein the electroactive ceramic is selected from the group consisting of piezoelectric ceramics and electrostrictive ceramics.
 7. The method of claim 1, wherein poling the electroactive ceramic precedes laser machining.
 8. The method of claim 1, wherein poling the electroactive ceramic follows laser machining.
 9. The method of claim 1, wherein the electroactive ceramic comprises grooves which render its electromechanical properties anisotropic.
 10. The method of claim 1, wherein at least 1% of the electroactive ceramic is removed during laser machining.
 11. The method of claim 10, wherein at least 5% of the electroactive ceramic is removed during laser machining.
 12. The method of claim 10, wherein at least 20% of the electroactive ceramic is removed during laser machining.
 13. The method of claim 10, wherein at least 50% of the electroactive ceramic is removed during laser machining.
 14. The method of claim 10, wherein at least 75% of the electroactive ceramic is removed during laser machining.
 15. The method of claim 10, wherein at least 90% of the electroactive ceramic is removed during laser machining.
 16. The method of claim 1, wherein the electroactive ceramic possesses a surface area at least 10% greater after machining than its surface area before machining.
 17. An electromechanical device, comprising a substantially planar electroactive ceramic member having grooves defined on a planar surface of the member, whereby the grooves allow the member to conform to a curved surface.
 18. The electromechanical device of claim 17, wherein the device is an electromechanical sensor or actuator.
 19. The electromechanical device of claim 17, wherein the device can conform to a curved surface having a radius of curvature no greater than 0.25″.
 20. The electromechanical device of claim 17, wherein the grooves are substantially parallel and the member can conform to a cylindrical surface.
 21. The electromechanical device of claim 17, wherein the grooves are substantially concentric and the member can conform to a spherical surface.
 22. An electromechanical device, comprising a substantially planar bimorph electroactive ceramic member having slots defined in the member, whereby the slots multiply an electromechanical bending response of the bimorph member.
 23. The electromechanical device of claim 22, wherein the device is an electromechanical sensor or actuator.
 24. The electromechanical device of claim 22, wherein the slots are substantially concentric.
 25. The electromechanical device of claim 22, wherein the slots are substantially parallel. 